The present invention relates in general to board level transmission line drivers and receivers, and in particular, to simultaneous bi-directional drivers and receivers that allow both ends of the line to transmit without protocol.
Digital computer systems have a history of continually increasing the speed of the processors used in the system. As computer systems have migrated towards multiprocessor systems, sharing information between processors and memory systems has also generated a requirement for increased speed for the off-chip communication networks. Designers usually have more control over on-chip communication paths than for off-chip communication paths. Off-chip communication paths are longer, have higher noise, impedance mismatches and have more discontinuities than on-chip communication paths. Since off-chip communication paths are of lower impedance, they require more current and thus more power to drive.
In an attempt to increase the bandwidth and in some cases simplify the protocol of off-chip networks, designers have incorporated simultaneous bi-directional (SBIDI) communication drivers and receivers. In SBIDI data transmission, data may be transmitted from each end of a transmission line simultaneously much like a full duplex telephone network where both parties can talk at the same time. Since more than one binary source is transmitting, the signal on a transmission line must have more than two levels and the signal generally has three levels corresponding to when both sources are transmitting a zero, either source is transmitting a one, and both sources are transmitting a one. In a network with a limited voltage swing, this results in an expected reduction in signal levels for differentiating each particular data stream. This reduction in signal levels may also result in a reduction in the signal to noise ratio. However, the SBIDI systems are able to transmit twice the amount of data over the same transmission line. In a system with well controlled transmission lines, SBIDI signaling may be a good design choice.
Off-chip communication paths may have multiple discontinuities. A signal originating at an on-chip driver traverses one impedance path from the driver to the chip I/O, another impedance path from chip I/O to the chip carrier I/O and yet another path within a circuit board. To get to its final destination, an off-chip signal may also have to traverse connectors and then paths in the packaging of a receiving chip. At high speed off-chip communication frequencies, the reflections and noise couplings may reduce SBIDI signaling reliability. In this case, the designer may have to revert to unidirectional (UNI) signaling to get the higher signal swings and improved signal to noise ratio.
System designers like to have one type of off-chip communication circuitry that may be used in a variety of off-chip networks without having to design special drivers and receivers. Since there are times SBIDI when signaling is appropriate and other times when UNI signaling is appropriate, there is a need for a transceiver design for off-chip networks that allows the system designer to switch a transceiver from SBIDI to UNI depending on the quality of the network without having to re-wire to different circuitry.
A simultaneous bi-directional (SBIDI) driver has current source circuits for delivering controlled amounts of current to a transmission line depending on logic gating signals. The near end of the transmission line is connected to one SBIDI driver and the far end is connected to another SBIDI driver. The transmission line has near and far end terminators comprising two resistors connected in series across the transmission line with the common node of the series connection coupled to one half of the power supply voltage. A SBDI receiver has a comparator section which generates an output in response to the difference voltage on its positive and negative inputs. The positive input is generated as the output of a first summing network and the negative input is generated as the output of a second summing network. A SBDI receiver is coupled to the near and far ends of the transmission line.
An additional SBIDI driver is used at the near and far ends as a replica driver whose output is coupled to a resistor terminator network like the transmission line terminator. The first summing network of the SBIDI receivers is coupled to the positive side of the transmission line and the negative side of the output of the replica driver generating the sum of the corresponding two signals. The second summing network of the SBIDI receivers is coupled to the negative side of the transmission line and the positive side of the output of the replica driver generating the sum of the corresponding two signals. The near end SBIDI receiver subtracts the signal generated by the near end driver from the composite signal from the near and far end signals arriving at the near end resulting in the near end SBIDI receiver detecting the far end transmitted data. Likewise, the far end SBIDI receiver subtracts the signal generated by the far end driver from the composite signal from the near and far end signals arriving at the far end resulting in the far end SBIDI receiver detecting the near end transmitted data. Both the near and far end SBIDI drivers and replica drivers have enable signals which function to turn the SBIDI replica drivers OFF and selectively set the SBIDI driver outputs into a high impedance mode (tri-state).
Both the near and far ends of the transmission line have a unidirectional (UNI) receiver coupled to the transmission line. If the transmission line is such that the reliability of SBIDI signal transmission is questionable, the UNI mode may be enabled. In the UNI mode, the replica SBIDI drivers are gated OFF and the SBIDI drivers are selectively gated OFF and ON depending on which end of the transmission line is sending or receiving data. The SBDI receivers are gated OFF when the UNI receivers are gated ON. In the UNI signal transmission mode, the magnitude of the current sources may be modulated by controlling how many of the current source circuits in each SBIDI driver is ON during a data bit cycle. The outputs of the SBIDI and UNI receivers are logic OR""ed together to generate the near end and far end detected data signals.
The present invention results in an electronically controllable driver/receiver system for data transmission lines that allows a designer to select the mode of operation best suited to the transmission line system while keeping one common circuit topology.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.